The transmission capacity of optical communications systems is presently limited by the optical source modulation bandwidth and dispersive and nonlinear propagation effects. Although the optical fiber has a very broad optical bandwidth (10-20 THz), the system data rates transmitted over fibers are presently limited to about 2.5 Gbits/sec for single-channel communications systems using typical single-channel communications approaches with conventional sources such as wavelength-tuned distributed-feedback (DFB) lasers. Wavelength division multiplexing (WDM) generally increases optical system capacity by simultaneously transmitting data on several optical carrier signals at different wavelengths. The total system capacity is increased by a factor equal to the number of different wavelength channels. Other advantages of WDM are in point-to-multipoint communications systems such as in fiber-to-the-home. In this case, improved power splitting budget, security, upgradability, service flexibility and lower component speed requirements compared to time-division-multiplex (TDM) point-to-point links make WDM attractive.
As used herein, the term "WDM" system will refer generally to a system capable of transmitting data on several wavelength channels. Other systems may use a number of individual optical modulated sources tuned to different wavelengths, and then combined and transmitted together.
Prior art WDM systems which transmit data on many channels, therefore generally include a separate optical modulation source for each channel. For example, an array of laser diodes may be used with each laser diode tuned to a different frequency and individually modulated. The laser frequencies are generally evenly spaced, combined using an optical coupler and then transmitted through an optical fiber. At the other end of the fiber, a device is used to separate the wavelength channels, and a separate optical receiver is generally used for each of the wavelength channels.
Despite the substantially higher bandwidth in fiber-based communications schemes that could be obtained with a WDM approach, present WDM suffer from a number of difficult technical problems, and at present WDM systems are not commercially viable for mass market applications such as fiber distribution to the home. For example, WDM systems would be most cost-effective for a large number of channels (32-64 or even 128), however present multi-channel laser diodes are very difficult to fabricate with acceptable yield even with a few as 8 channels. In addition, passive WDM splitters currently available have a large temperature variation of their passband channels, thereby requiring continuous tunability in the multichannel sources which is not yet available. Packaging and complexity/yield problems with current WDM systems approaches thus represent a significant problem in present WDM systems. These complexity and yield problems significantly increase the cost of the WDM implementation.
Therefore, although WDM offers an elegant solution to increasing the capacity and transparency of optical networks, WDM for fiber distribution networks as currently envisioned is not cost-competitive with simple point-to-point schemes (one fiber per customer), and more cost-effective schemes are needed. For fiber-to-the home optical communications systems, low-cost methods of delivering optical signals into and out from the home is a challenging problem. Although time-domain multiplexing (TDM) of data streams would be another method of increasing transmission capacity, it is not desirable to build a specific network with expensive high frequency electronic components that are difficult to upgrade in the future. For example, in order to deliver 50 Mbits/sec data rates into a single house, a 32 channel system would require transmitters, routers, amplifiers, receivers and modulators with 1.5 Gbits/sec capacity and above. It is not desirable to place such expensive and state-of-the-art components into every home. In addition, it is desirable to have as much of the system in the field and in the home transparent and passive, i.e. line-rate independent and not requiring any electrical powering. In addition to the low data rate systems as required for local access (50-155 MHz), high data rate systems (622 MHz-2.5 Gbits/sec) can also benefit from WDM. In such a case, similar problems are caused by the difficulty in obtaining a multifrequency source with adequate channel tuning, stability and modulation bandwidth.
As is apparent from the above, there is a continuing need for an efficient and cost-effective WDM system that is capable of transmitting a large number of spectral channels.